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Bacteria in the Gut Linked to Obesity


The bacteria living in your gut thrive on fat and sugar, and so when they get the food they want and need, their good fortune can result in obesity. That is what researchers at Washington University School of Medicine in St Louis discovered when they transplanted human intestinal (gut) bacteria into germ-free mice and fed them a junk food diet.

The study, which was published in Science Translational Medicine, involved the use of humanized mice, which means scientists were able to recreate the environment of the human gut in the laboratory animals. This allows researchers to control the many variables that are involved in studying obesity and nutrition in humans.

In this study, the researchers selected a group of mice and fed them low-fat, plant-rich diets in the weeks before they underwent the transplant that would turn them into humanized mice. The mice continued to consume the same diet after the transplant as well. Stool samples were collected and analyzed one day, one week, and one month after the transplants took place.

One month after the transplants, half the mice were switched to a high-fat, high-sugar diet, resembling the “western” way of eating. Stool samples were collected and analyzed from all the mice 24 hours after the diet change and then again weekly for two months. The researchers noted a change in the microbial environment of the mice on the high-fat, high-sugar diet as soon as 18 to 20 hours after their exposure to the western diet.

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Compared with the mice who continued to eat the low-fat, plant-based diet, mice on the western diet had a significantly greater proportion of two types of gut bacteria that belong to a group (phylum) called Firmicutes. They also noted a reduction in bacteria in another phylum known as Bacteroidetes. These are changes found in earlier studies to link obesity in mice and humans.

Further analysis revealed that the mice on the western diet had more microbial genes devoted to breaking down and processing simple sugars and other factors found in a high-fat, high-sugar diet. They also discovered that these genes were activated in the mice consuming the unhealthy diet.

The bottom line, based on this study and previous ones, appears to be that gut bacteria have a powerful effect on how the body digests food and breaks it down into energy. Research results indicate that Firmicutes, which thrive in a high-fat, high-sugar environment, rapidly populate the gut when the diet is changed from a healthy low-fat, plant-based approach to a western diet.

According to Jeffrey I. Gordon, MD, director of Washington University’s Center for Genome Sciences and senior author of the study, additional research could lead to a better understanding of how the body is likely to respond to calories. Gordon and his colleagues are already using the humanized mouse model to find ways to fight malnutrition in children. “By analyzing the microbial communities, we hope to identify the microbial deficiencies that explain why some children and not others suffer from malnutrition,” he said in the University’s news release.

Time magazine Nov. 12, 2009
Washington University in St. Louis, Nov. 11, 2009 news release